1. Field of the Invention
This invention relates generally to electropolishing processes, and more particularly to a system and method for electropolishing the inner surface of a pipe. Even more particularly, the present invention relates to a system and method for maintaining process parameters (e.g., the electrode-pipe gap voltage within acceptable limits.
2. Description of the Background Art
FIG. 1 is a block diagram of a typical electropolishing system 100 for polishing the inner surface of a pipe 102. Electropolishing system 100 includes a power a power supply 104 having a first voltage supply terminal 106 and a second voltage supply terminal 108, a cable 110, an electrode 112, and an electrical lead 114. Electrode 112 is coupled to first voltage supply terminal 112 via cable 110, and pipe 102 is coupled to second voltage supply terminal 108 via electrical lead 114. An electrolyte solution (not shown) is circulated through pipe 102 during the electropolishing process by an electrolyte pumping system (not shown).
Power supply 104 asserts a first voltage, via first voltage supply terminal 106 and cable 110, on electrode 112, and a second voltage, via second voltage supply terminal 108 and electrical lead 114, on pipe 102. The voltage difference between electrode 112 and pipe 102 causes electrical current to flow from electrode 112, through the electrolyte solution (e.g., phosphoric acid or sulfuric acid solution), to pipe 102. The electrical current selectively removes microscopically raised points from the inner surface of pipe 102 (including any deposits thereon), into the electrolyte solution in the form of a soluble salt, effectively polishing the inner surface of pipe 102.
The amount of electrical current, and thus the amount of material removed from the inner wall of pipe 102, depends on the voltage difference between electrode 112 and pipe 102. When electrode 112 is in position W, close to the connection between pipe 102 and lead 114, the resistance of pipe 102 is negligible, so that the voltage difference between electrode 112 and pipe 102 is essentially equal to the voltage difference between first supply terminal 106 and second supply terminal 108. However, during the electropolishing process, electrode 112 is drawn through pipe 102 from position W to position X, to position Y, and finally to position Z. As electrode 112 is drawn away from position W the resistance of pipe 102 becomes a factor, reducing the voltage difference between electrode 122 and pipe 102. The changing voltage between electrode 112 and pipe 102 results in the nonuniform electropolishing of the inner surface of pipe 102.
FIG. 2 is a chart 200 showing the increased resistance of pipe 102, and the associated change in voltage across the electrolyte gap, as electrode 112 is drawn through pipe 102. The electrode positions (W, X, Y, Z) shown in chart 200 correspond to the positions illustrated in FIG. 1. The resistance (R) of pipe 102 was determined by the equation: EQU R=.rho.(D/A), (Eq. 1)
where R is the resistance of pipe 102, .rho. is the resistivity of the material of which pipe 102 is constructed, L is the length of pipe between the particular position and position W, and A is the cross-sectional area of pipe 102. For purposes of this example, .rho. is taken as 2.362 micro-ohms per foot, the resistivity of type 304 stainless steel, and the cross-sectional area of pipe 102 is 0.0655 square feet. The voltage drop (V) through a particular section of pipe 102 is calculated using Ohm's law: EQU V=IR, (Eq. 2)
where (I) is the process current and (R) is the resistance of the relevant section of pipe 102.
The voltage between electrode 112 and pipe 102 (the gap voltage) is determined by subtracting the voltage drop across the particular pipe section from the voltage between first supply terminal 106 and second supply terminal 108 (the process voltage). Chart 200 shows that as the distance between electrode 112 and the junction between pipe 102 and lead 114 (position W) increases, the gap voltage decreases. The decreasing gap voltage results in the nonuniform electropolishing of the inner surface of pipe 102.
What is needed is an electropolishing system and method, wherein the gap voltage may be maintained within a desired range to achieve the uniform electropolishing of the inner surface of a pipe.